1. Field of the Invention
The invention relates to radioactive therapeutic seeds. More particularly, the invention relates to improved radioactive therapeutic seeds for the treatment of oncological and other medical conditions.
2. State of the Art
Radioactive seed therapy is a well known and well accepted medical procedure for the treatment of various oncological and other medical conditions. Seed therapy, also known as brachytherapy typically involves the implantation of fifty to one hundred tiny capsules (seeds) into or around a treatment site. The capsules contain a radioactive isotope which irradiates the treatment site at close range without adversely affecting other parts of the body. Brachytherapy has been used successfully in the treatment of various types of cancers such as prostate cancer. It has also been used to prevent the growth or regrowth of tissues in the treatment of various occlusive diseases such as arteriosclerosis and arthrosclerosis subsequent to balloon angioplasty.
Radioactive therapeutic seeds are carefully designed to possess several important qualities. First, they are relatively small, typically approximately 0.025 inch in diameter and approximately 0.16 inch long so that they may be implanted using minimally invasive instruments and techniques. Second, the radioactive isotope must be enclosed in a biocompatible protective package since the seeds are typically not removed and will remain in the body for many years. Third, each seed preferably includes a radiopaque (e.g. high Z material) marker so that it can be located at the treatment site with the aid of fluoroscopy. Fourth, the protective package and the radiopaque marker have been designed to not cast "shadows" in the irradiation pattern of the isotope. Fifth, the isotope has been evenly distributed within the protective package so as to avoid any "hot spots" of radiation.
The state of the art of radioactive therapeutic seeds is substantially disclosed in seven U.S. Pat. No. 5,713,828 to Coniglione for "Hollow-Tube Brachytherapy Device", U.S. Pat. No. 5,405,309 to Carden, Jr. for "X-Ray Emitting Interstitial Implants", U.S. Pat. No. 4,891,165 to Suthanthiran for "Device and Method for Encapsulating Radioactive Materials" and U.S. Pat. No. 4,784,116 to Russell, Jr. et al. for "Capsule for Interstitial Implants", U.S. Pat. No. 4,702,228 to Russell, Jr. et al. for "X-Ray Emitting Interstitial Implants", U.S. Pat. No. 4,323,055 to Kubiatowicz for "Radioactive Iodine Seed", and U.S. Pat. No. 3,351,049 to Lawrence for "Therapeutic Metal Seed Containing within a Radioactive Isotope Disposed on a Carrier and Method of Manufacture".
The Lawrence patent describes many of the essential features of radioactive therapeutic seeds. Lawrence describes radioactive isotopes (I-125, Pd-103, Cs-131, Xe-133, and Yt-169) which emit low energy X-rays and which have relatively short half-lives. When implanted at a treatment site, these isotopes provide sufficient radiotherapy without posing a radiation danger to the medical practitioner(s), people in the vicinity of the patient, or other parts of the patient's body. Lawrence further describes a protective capsule which contains the isotope and prevents the isotope from migrating throughout the body where it might interfere with healthy tissue. The capsule is cylindrical and made of low atomic number biocompatible materials such as stainless steel or titanium which substantially do not absorb X-rays. The isotope is coated on a rod shaped carrier made of similar X-ray transparent (e.g. low Z) material, placed inside the capsule cylinder along with a radiopaque marker wire, and the ends of the capsule cylinder are closed.
In 1980, Kubiatowicz made a minor improvement in the basic Lawrence design by providing that the entire isotope carrier be made of radiopaque material such as silver. Kubiatowicz recognized that since the isotope was carried on the entire outer surface of the carrier, there was no need to make the carrier body X-ray transparent as suggested by Lawrence. The larger radiopaque carrier body described by Kubiatowicz makes the seeds easier to see with X-ray or fluoroscopic examination.
Several years later, Russell, Jr. et al., in U.S. Pat. Nos. 4,707,228 and 4,784,116, explained that the capsule design of Lawrence and Kubiatowicz produces anisotropic angular radiation distribution. According to Russell, Jr. et al., the shell forming techniques used in the Lawrence-type seeds result in large beads of shell material at the ends of the seeds. These beads substantially shield radiation thereby casting undesirable shadows in the longitudinal ends of the irradiation pattern of the isotope. Russell, Jr. et al. proposed a new seed design to solve this problem. In particular, Russell, Jr. et al. proposed a seed having a cylindrical container which is sealed with end caps which have a wall thickness that is substantially the same as the wall thickness of the cylindrical container. The end caps are attached to the cylindrical container by welding or crimping.
An alternate solution to the non-uniform radiation pattern of the Lawrence type seeds was proposed by Suthanthiran in U.S. Pat. No. 4,891,165. Suthanthiran's solution was to form a seed capsule from two interfitting sleeves, each having one open end and one closed end. The thickness of the sleeve side walls and their closed ends is such that when the sleeves are interfit in an overlapping manner, the total side wall thickness of the assembled capsule is approximately equal to the end wall thickness.
Despite the fact that radioactive therapeutic seeds have been in use for over thirty years and despite the several significant improvements made in these seeds, many concerns still exist regarding their design and construction.
Due to the general manner in which brachytherapy seeds are implanted, there exists the possibility that the seeds may migrate during implantation and post-operatively. Seeds are typically delivered to a treatment site by loading a plurality of seeds into a needle having an open distal end and an obturator extending into the proximal end of the needle proximal of the seeds. The needle must be handled with care to prevent the seeds from falling out of the open distal end of the needle as they are inclined to do if the distal end of the needle is tilted downward. The needle is then inserted into the patient to the farthest implant site. The needle is then slowly withdrawn while maintaining the obturator at its current location such that the needle moves proximally relative to the obturator and the seeds. The seeds are thereby released from the needle at the intended location. This difficult maneuver often results in some proximal movement of the obturator relative to the implant site which creates a negative pressure between the obturator and the seeds. The negative pressure can unintentionally draw properly positioned cylindrical seeds away from their intended implant site. Additionally, depending upon the locus of implantation, seeds may also unintentionally migrate after the implantation procedure. The consequences of unintended seed movement can be serious. First, the tissue requiring treatment may not receive the prescribed radiation dose. Second, healthy tissues may be dosed with radiation to the extent that the healthy tissues become damaged. Therefore, there is an important, yet unresolved, issue with respect to immobilizing a seed at the desired implant site.
In addition, as discussed with respect to the patents of the prior art, the art of brachytherapy seeds has had a longstanding focus on creating seeds having an isotropic radiation distribution; i.e., a longitudinally uniform radiation distribution. This is not surprising, particularly because the physician has had no way of controlling the rotational orientation of the seed in the body of the patient. Moreover, even if rotational control were not a problem, no prior art has addressed the value of a radially non-uniform radiation distribution. However, if it were possible to control the orientation of a seed within the body and ensure that the seed did not unintentionally move radially or axially about its longitudinal axis after implantation, a seed purposefully configured for a radially non-uniform radiation distribution would be a valuable tool in brachytherapy treatment.